The invention relates to a method for encapsulating an electronic component, in particular a semiconductor chip on a substrate. Such methods permit the manufacture of so-called chip scale packages (CSP) with which the completed semiconductor element provided with connection parts is only insignificantly larger than the bare silicon chip.
With this the basic idea lies in that between the substrate and the component there is arranged a buffer mass, for example of a silicone elastomer, which compensates the coefficient of thermal expansion differences of the materials. With this also the soldered connections of the installed semiconductor chip are only subjected to slight stress during environmental temperature fluctuations. The life duration and reliability of such chip scale packages is thus relatively high. Known manufacturing methods are for example described in xe2x80x9cSemiconductor Internationalxe2x80x9d, November 1997, page 48, or in U.S. Pat. No. 5,659,952 or U.S. Pat. No. 5,679,977. In spite of the improved properties of the end product, the known methods are however relatively complicated and do not permit an efficient manufacture. In particular there are necessary numerous individual steps with a comparatively high cost with respect to required apparatus and time required for manufacture. It is therefore an object of the invention to provide a method of the above mentioned type with which fewer processing steps are required. Furthermore with the placing of the component onto mechanical installation parts and/or onto the component itself there is to be effected no unnecessary exposure to heat because micromechanical processes with a simultaneous heat effect are difficult to handle and may tend to degrade the electronic component.
This object according to the invention is achieved with a method in which the buffer material and/or an adhesive in a liquid or pasty (flowable form is deposited from a dispenser onto a substrate. With this at least one working step which was necessary until now may be saved, specifically the pressing on of an adhesive in a stencil printing method. The component itself is at room temperature placed onto the buffer material and/or onto the adhesive and not as previously in the heated condition. With this the term xe2x80x9croom temperaturexe2x80x9d is to be understood as a temperature which prevails in working rooms corresponding to the applicable standards in air conditioning technology and which settle roughly in the temperature range from 10xc2x0 C. to 45xc2x0 C., preferably from 16xc2x0 C. to 26xc2x0 C. With this the throughput per unit of time is improved since no heating time for the component is required. Mechanical installation parts and/or measuring instruments are not compromised by the effect of heat.
After the placing-on of the component the buffer material and/or the adhesive may be subjected to a precuring and a curing and specifically advantageously on the same installation as the placing-on of the component. By way of this in a particularly advantageous manner there is exploited the condition that with the adhesives applied here, the joining together of the parts to be connected and the curing of the adhesive do not necessarily have to coincide. By way of the temporal separation, the precuring and/or the curing may be effected in an installation region at which heat development is less problematic. After the precuring or curing the connected units without risk of a mutual displacement of the substrate and component may be transported to any other working station.
An essential process simplification may be achieved in that firstly several buffer cams or nubbins are applied to a substrate such as in a stencil printing method, the buffer cams are cured. Before the placing-on of the component, adhesive is deposited with the dispenser on or next to various buffer cams. With this method existing stencil printers for depositing the buffer cams (nubbins) may be used. In contrast the despositing of the adhesive is effected in a considerably more simple manner with the dispenser. Advantageously adhesive is deposited simultaneously from several dispenser openings, each to one of a plurality of nubbins. Under certain circumstances a dispenser, which in rapid succession coats a group of buffer cams with an adhesive would however also be conceivable.
With a suitable material choice the dispenser may be used to deposit the buffer cams may be deposited onto the substrate and be secured thereto. Subsequently each buffer cam may be provided with an adhesive layer. This may either be effected with a separate dispenser or with the same dispenser.
A further essential simplification of the process may finally be achieved in that an adhesive material may be applied that simultaneously serves as a buffer material. With this the subdividing into two separate materials is done away with, which simplifies the procedure as a whole. The adhesive may with this be deposited in the form of individual buffer bodies, which remain alone after the placing-on of the component. Subsequently, the voids are together with the covering of the wiring locations, filled out with a protective mass. Alternatively however it is also conceivable that the adhesive, with the dispenser, is deposited as an adhesive pattern which after placing on the component connects to a homogeneous layer between the component and the substrate. Air pockets are thus prevented and a retrospective filling out of the voids is no longer necessary. Such adhesive patterns are already known in semiconductor encapsulation technology.
Further advantages may be achieved when the depositing of the buffer material and/or of the adhesive, with the dispenser, is effected directly shortly before the placing on of the component at the same machine. Advantageously with this also the substrate, on the same transport system, is led past the dispenser and the placing-on device for the component. On the one hand with this both working steps may be exactly matched to one another and the drying or curing time may be better controlled. In the same machine and on the same transport system a smooth transport is possible, by which means the danger of an inadvertent displacement of the component from the nominal position is greatly reduced. Before the placing on the component is advantageously adjusted with respect to position.
An improvement of the planarity of the component and the substrate may be achieved in that during the placing on of the component the substrate is secured on a rest surface by way of a vacuum.
For improving the manufacturing rationality advantageously groups of components are fastened on the same substrate. For this in each case several components after one another are deposited to a group. The whole group may then simultaneously be subjected to a pressing-on pressure and/or to a heat treatment. This pressing on and/or heating in groups may also be applied with alternative encapsulation processes in which the components are not deposited at room temperature. The simultaneous processing of the components may be effected via commonly actuated individual tools, such as e.g. individual pressing punches, which however are actuated simultaneously. Alternatively it may also however be the case of a common tool which impinges the whole group simultaneously, such as e.g. a common pressing punch of a suitable size.
Generally, in order to ensure an adequate adhesion and to cure the elastomer mass, the components with the simultaneous heating must be pressed against the substrate.
A disadvantage of the known method lies however in the fact that the exact placing of the components on the substrate and the final connection under a high pressure and at a high temperature is effected in one step. The pressing procedure with this lasts considerably longer that the placing procedure which leads to great capacity reductions in the production line. On the other hand however the application of high temperatures in the region of the placing of the components is disturbing in that the heat may damage the component or the adhesive materials. At the placing station, specifically highly precise manipulators and measuring apparatus operate, which react sensitively to temperature effects. It is therefore a further object of the invention to provide a method of the previously mentioned type with which, with simple means, the production capacity within the same line may be increased. Furthermore the method is to permit a separation of the placing-on procedure from the actual processing procedure for the definitive connection.
This object, according to the invention, is achieved with a method which In which the pressing in groups and/or heating of the components is with this effected separately with respect to time from the placing-on procedure so that according to the tool size a relatively large number of components may be simultaneously treated.
With this, the placing of the components onto the substrate and the impinging with the tool at spaced working stations are effected particularly advantageously. Specifically it has been shown that the exact placing and the processing do not necessarily have to be effected at the same working station. With the application of suitable substrates and adhesives it is possible to achieve adequate adhesive force at room temperature for the further transport after the placing. With this the substrates may be transported away from the sensitive placing region to a processing station at which without negative influence to the placing procedure, pressure and/or heat may be applied.
Several groups of components after one another may with this in a cycled manner be impinged, wherein the impingement of a group in the ideal case lasts maximally for as long as the placing-on of a preceding group. With the placing-on procedure with this in a rapid succession the components may be arranged to a group, wherein at the processing station during the whole time duration from the placing of the first component up to the placing of the last component one presses. Considerable advantages with the processing of the components may further be achieved when each component is impinged with a separate plunger and when with the impinging for achieving a uniform bearing force on all components each plunger is adapted to the individual height of the component. With this measure it is ensured that also with the unavoidable height tolerances of the component a uniform processing of the whole group with pressure and temperature is effected. A component with a maximum height which is only just allowable will thus be subjected to the same pressing force and to the same temperature as a component with an allowable minimum height.
For the heat treatment before or during the impingement of the components the plunger and/or a substrate rest serving as a support are heated. It would however also be conceivable to carry out the whole pressing procedure in a closed oven or to effect the heating in another manner, e.g. by way of a convection oven or by microwaves. The substrate is furthermore during the impingement of the components advantageously held on the substrate rest by way of a vacuum. Since with the substrate it is usually the case of a film-like or gel-like material, with this also a flat lying on the substrate rest is ensured without air pockets.
The invention also relates to a device for carrying out the described method. With slight constructional adaptations with this advantageously known equipping automatic machines (die bonders) may be applied, as is known e.g. by way of WO 97/32460, the contents of which are hereby incorporated herein in its entirety.
The dispensing station may comprise a dispenser with several delivery openings or however a dispenser with at least one movable dispenser opening with whose help by way of a simultaneous movement and delivery of the means a certain pattern is drawn.
A particularly compact manufacturing unit may be achieved when in the transport direction after the placing-on station there is arranged a precuring station and/or a curing station in which the unit consisting of the substrate and component is impingable by way of electromagnetic rays, in particular by way of infrared rays or ultra-violet rays. The semiconductor components may run through this precuring station or curing station in the same operating cycle as the dispensing station and placing-on station.
The invention also relates to a device for processing components arranged on a substrate, in particular semiconductor chips. This device is particularly suitable for carrying out the method according to the invention, but may also be applied in other manners. The device in particular is to ensure that in spite of the processing of the components in a group, the individual nature of each component is taken account of as with an individual processing.
The individual and displaceably mounted plungers in the tool permit each individual component of a group to be processed individually. Conceivable therefore would be the exertion of an individual pressing force and/or an individual temperature. The individual plungers may be shaped differently so that within a group various sizes and shapes of components may be impinged.
With the above mentioned manufacture of semiconductor elements however as a rule only the differing heights of the raw silicon chips need be taken into account. This is preferably effected in that the plungers are in active connection with a compensation means. This compensation means may function according to differing technical principles. Thus for example a purely mechanical solution would be conceivable, with which in a neutral home position of the tool the displaceable plungers are applied loosely onto the components. Via a central locking then all plungers are locked in their sliding bearing whereupon via the tool the pressing force is exerted via the individually adjusted plungers. It would however also be conceivable to allocate to each individual plunger a pressure means cylinder whose allowable inner pressure may be individually controlled.
In a particularly simple manner the compensation means however is formed by a fluid chamber or manifold for the hydrostatic distribution of an equal impingement force onto the individual plungers. The fluid chamber functions according to the known principle, according to which in a practically non-compressible still fluid, the pressure forces propagate independent of direction. With this each of the plungers could with an equal impingement surface submerse into the fluid chamber whose walls are otherwise rigid.
A technically considerably more simple solution lies in arranging the fluid chamber on the rear side, of the plunger, which is distant to the substrate rest and for it to comprise a flexible membrane, wherein the rear sides of the plunger bear on the membrane. The volume in the chamber always remains the same size, independently of how the plungers are positioned. With this the elasticity of the membrane within a certain tolerance region permits a displacement of the plungers. The pressure in the fluid and thus the pressure on the individual plungers with this however remain constantly equal.
For monitoring and control or where appropriate also for recording the pressing force acting on the plungers the fluid chamber may be in active connection with a pressure sensor. It is also with tools without fluid chambers generally advantageous to control the pressing pressure of the tools via a pressure sensor, in order to approximate the pressing force during the whole pressing duration to a nominal value. In particular pressure sensors, but also other sensors, e.g. force sensors may be applied.
The plungers could be mounted in a plunger guide, which for heating the plunger is provided with a heating device. The plungers may with this, indirectly via at least one heating device, be heated to process temperatures, for instance of up to 300xc2x0 C. Preferably the plunger guide and the plunger consist of the same metallic material (or materials having compatible coefficients of thermal expansion) so that no jamming occurs with the thermic expansion. The plungers specifically must be guided in the plunger guide with a very small play so that exclusively vertical forces and no transverse forces act on the working surface of the plungers. For the same reason the plunger guides in relation to the diameter of the plungers are preferably formed relatively long.
In order to prevent the irradiation of heat or to protect the operating personnel, the tool at least partly may be surrounded by a heat-insulating layer. The same also applies to the substrate rest in the case that this is likewise heated by way of a heating device. So that also the operating temperature may be monitored and controlled in the tool as well as in the substrate rest there is arranged at least one temperature sensor.
For safety reasons the plungers in the idle condition of the tool are biased into a neutral home position in which they are preferably completely retracted into the tool. In this manner the plungers may not be damaged when the tool for example for re-equipping work is removed from the tool mounting. Furthermore it is thus ensured that the plunger rear sides in the home position bear uniformly on the membrane of the fluid chamber. Finally with the spring biasing it is also preferably ensured that the plungers for the optimal heat accommodation after each operating stroke again completely submerge into the plunger guide.
The substrate rest rests preferably on an adjustable three-legged stand. By way of this the substrate rest in a particularly simple manner may be aligned plane-parallel onto the working plane of the plungers. Also this measure serves the prevention of transverse forces acting on the components.
The device may in particular be used for re-pressing the chips in a line for manufacturing chip scale packages (CSP), in particular ball grid arrays (BGA) or flex BGA""s. With the same devices however also other processings in semiconductor manufacture may be carried out. Thus for example in the field of flip chips, wherein the chips are adhered with bumps, are eutectically fastened or are soldered. The device is however also suitable for LOC processes (lead on chip) with which the connection between the chips and a metallic lead frame is manufactured under pressure and temperature. Finally it would however also be conceivable to apply the device according to the invention also outside semiconductor technology, for example in the field of electrotechnology or opto-technology.